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开发环境：VS2019，.NET Core 3.1，ASP.NET Core API

1、建立一个验证码控制器

新建两个方法Create和Check，Create用于创建验证码，Check用于验证它是否有效。

声明一个静态类变量存放列表，列表中存放包含令牌和验证码的对象。

        private static List<VCodeModel> _list = new List<VCodeModel>();/// <summary>/// 返回一个图形验证码和令牌./// </summary>/// <returns></returns>/// <remarks>这一步需要生成一个数字，并将数字放入图片中，并返回一个令牌，下次验证的时候需要核对令牌和数字是否对应，这些东西保存到类变量中.</remarks>[HttpGet]public string Create(){try{VCodeModel model = new VCodeModel();model.id = Guid.NewGuid().ToString();    // 生成令牌var vcode = Utils.GetVCode();    // 生成验证码model.code = vcode;_list.Add(model);    // 记录到缓存中// 返回对象VCodeController_Create_Receive result = new VCodeController_Create_Receive();result.code = "0";result.data = new VCodeController_Create_Receive.Data();result.data.id = model.id;var image = Utils.DrawImage(vcode);var base64 = Utils.BitmapToBase64Str(image);result.data.img = base64;var json = JsonConvert.SerializeObject(result);return json;}catch (Exception ex){_logger.LogError(exception: ex, message: ex.Message);ReceiveObject result = new ReceiveObject();result.code = "999999";result.msg = "系统异常";var json = JsonConvert.SerializeObject(result);return json;}}/// <summary>/// 检查验证码是否有效/// </summary>/// <param name="id">令牌.</param>/// <param name="code">验证码.</param>/// <returns></returns>[HttpGet]public string Check(string id, string code){try{var index = _list.FindIndex(m =>{if (m.id.Equals(id) && m.code.Equals(code)){return true;}return false;});ReceiveObject result = new ReceiveObject();if (index >= 0){_list.RemoveAt(index);result.code = "0";result.msg = "验证成功";var json = JsonConvert.SerializeObject(result);return json;}else{result.code = "1";result.msg = "验证失败";var json = JsonConvert.SerializeObject(result);return json;}}catch(Exception ex){_logger.LogError(exception: ex, message: ex.Message);ReceiveObject result = new ReceiveObject();result.code = "999999";result.msg = "系统异常";var json = JsonConvert.SerializeObject(result);return json;}}

2、建立一个验证码模型

验证码模型类包括：令牌和验证码属性。

再创建一个类存放Create方法返回的对象包括：令牌，验证码图片。

    /// <summary>/// VCodeController用到的类变量./// </summary>public class VCodeModel{/// <summary>/// 令牌./// </summary>public string id { get; set; }/// <summary>///验证码./// </summary>public string code { get; set; }}/// <summary>/// VCodeController控制器中Create方法的返回对象./// </summary>public class VCodeController_Create_Receive : ReceiveObject{public Data data { get; set; }public class Data{/// <summary>/// 令牌./// </summary>public string id { get; set; }/// <summary>/// Base64的验证码图片./// </summary>public string img { get; set; }}}

3、建立一个验证码生成器
创建用于生成4位随机数的方法。

创建将随机数绘制成图片的方法。

        static readonly char[] list = new char[10] { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', };/// <summary>/// 获取随机验证码./// </summary>/// <returns></returns>public static string GetVCode(){Random random = new Random();StringBuilder builder = new StringBuilder();for (int i = 0; i < 4; i++){var index = random.Next(0, list.Length);builder.Append(list[index]);}return builder.ToString();}/// <summary>/// 绘制验证码的图片./// </summary>/// <param name="code"></param>/// <returns></returns>public static Bitmap DrawImage(string code){Color[] list_color ={Color.FromArgb(240, 230, 140),    // 黄褐色（亮）Color.FromArgb(138, 54, 15),    // 黄褐色（暗）Color.FromArgb(51, 161, 201),    // 蓝色（亮）Color.FromArgb(25, 25, 112),    // 蓝色（暗）Color.FromArgb(192, 192, 192),    // 灰白（亮）Color.FromArgb(128, 128, 105),    // 灰白（暗）};Random random = new Random();// 创建画板Bitmap bitmap = new Bitmap(150, 50);// 创建画笔Graphics grp = Graphics.FromImage(bitmap);grp.Clear(Color.White);    // 设置背景色为白色// 绘制噪点for (int i = 0; i < random.Next(60, 80); i++){int x = random.Next(bitmap.Width);int y = random.Next(bitmap.Height);grp.DrawLine(new Pen(Color.LightGray, 1), x, y, x + 1, y);}// 绘制随机的线条grp.DrawLine(new Pen(list_color[random.Next(list_color.Length)], random.Next(3)),new Point(random.Next(bitmap.Width / 2), random.Next(bitmap.Height / 2)),new Point(bitmap.Width / 2 + random.Next(bitmap.Width / 2), bitmap.Height / 2 + random.Next(bitmap.Height / 2)));// 绘制验证码for (int i = 0; i < code.Length; i++){var item = code[i];grp.DrawString(item.ToString(),new Font(FontFamily.GenericSansSerif, 25, FontStyle.Bold),new SolidBrush(list_color[random.Next(list_color.Length)]),x: (150 / 4) * i,y: random.Next(5));}// 在验证码上绘制噪点for (int i = 0; i < random.Next(30, 50); i++){int x = random.Next(bitmap.Width);int y = random.Next(bitmap.Height);grp.DrawLine(new Pen(list_color[random.Next(list_color.Length)], 1), x, y, x + 1, y);}// 绘制随机的线条grp.DrawLine(new Pen(list_color[random.Next(list_color.Length)], random.Next(3)),new Point(random.Next(bitmap.Width / 2), random.Next(bitmap.Height / 2)),new Point(bitmap.Width / 2 + random.Next(bitmap.Width / 2), bitmap.Height / 2 + random.Next(bitmap.Height / 2)));return bitmap;}/// <summary>/// 将图片对象转成Base64的字符串./// </summary>/// <param name="bitmap"></param>/// <returns></returns>public static string BitmapToBase64Str(Bitmap bitmap){using (MemoryStream memoryStream = new MemoryStream()){bitmap.Save(memoryStream, ImageFormat.Jpeg);byte[] bytes = memoryStream.ToArray();return Convert.ToBase64String(memoryStream.ToArray());}}

运行效果：

参考：C# 绘制验证码图片_c#生成验证码图片_耕耘虫的博客-CSDN博客